The invention relates to inks which are suitable for ink jet printers.
A continuous-jet ink jet printing device is described in U.S. Pat. No. 3,596,275 while a device of the "on demand" type is describe in Italian Patent No. 1 182 478 in the name of the applicant. A known variant of the "on demand" type is the so-called "bubble" or "thermal ink jet" type, which makes use of heat as a source of energy to generate a bubble of vapour which creates a brief high pressure pulse for the expulsion of a drop of ink. The latter type of printing device is described in U.S. Pat. No. 4,723,129, in which the drops of ink are expelled as a result of a strong pressure pulse created by the instantaneous formation of a bubble of vapour following the heating of a resistor placed in the ink duct upstream of the nozzle.
The inks which are employed in particular in bubble and thermal ink jet printers, must comply with specific requirements, such as the following:
the ink must produce images of high quality, in terms of definition and contrast, on plain paper; PA1 the ink must have good fixing characteristics on the paper, drying rapidly and proving itself to be difficult to remove by scrubbing; PA1 the ink must produce script with excellent storage characteristics, i.e. must give optimum performance in terms of "waterfastness" and of "lightfastness"; "waterfastness" is defined as the capacity of the script to resist the action of water, while "lightfastness" is defined as the capacity of the script not to lose its optical density, and not to change colour when it is exposed to the action of light; PA1 the ink must be entirely compatible with the materials of the writing head; PA1 the ink should not cause, even after prolonged periods of inactivity of the writing head, the nozzles to become clogged; PA1 the ink must be thermally stable; in particular, when it is used in a bubble or thermal head, it should not give rise to any insoluble combustion residues on the resistors; PA1 the ink must be stable over time, without exhibiting traces of sedimentation, nor of biological growth; PA1 the ink must be ecologically safe, nontoxic and nonflammable.
Among the above listed requirements, "waterfastness" and "lightfastness" of the script assume special importance where the indelibility of the document is required for legal use.
Inks which make use of dyes which are soluble in water, or in water-solvent mixtures, do not comply, in the present state of the art, in an entirely satisfactory manner with the requirements of waterfastness, lightfastness and of indelibility in general.
In order to solve this problem, various types of inks have been proposed, which provide, as an alternative to dyes in solution, the use of pigments.
Attention is drawn, by way of example, to Italian Patent No. 1 195 146 in the name of the applicant, as well as European Patent Applications Nos. 0 425 439 and 0 376 284. The Italian patent describes an ink in which the pigment is finely ground with dimensions not exceeding 1000 .ANG. and is contained in quantities of between 0.1% and 2.5% by weight of the ink.
The other two European patent applications relate to inks in which the dispersion of the pigment is obtained by means of carboxylic resins.
The instability of the suspension represents one of the more serious problems found in the formulation of inks which make use of pigments.
It has been desired to solve the problem of the instability of the inks which make use of suspensions of pigments providing, an ink in which the tendency towards the formation of microprecipitates is minimized.
It has also been desired to provide an ink which produces printed matter with excellent characteristics of waterfastness, lightfastness and, more generally, of indelibility.